Yes you can buy from the following link. After clicking "Buy" change from Full to Upgrade; then select your existing LR product from the drop-down menu and the correct pricing will be displayed. Then simply choose “Add to Cart”

Or you can take a 30 day free trial via the Creative Cloud to test what’s new (some new features are only available to CC subscribers) and after the trial it’s possible to convert the CC version to the LR6 stand-alone version, upon purchase of a serial number and without any further installation. In either case make sure your computer meets the minimum system requirements.

Your chaoices are to purchase a serial number for LR 6 and wait until LR 7 to get new feature updates, or subscribe to the Photograph Plan which includes PS-CC and LR-CC for US $10/month which will get new features updates right away as soon as they are available, with the first being Dehaze in LR CC 2015.1. Both LR 6 and LR CC 2015 will get bug fixes and new camera updates.